[DistGB] remove toindex() as torch tensor is always be expected

[Rhett-Ying]

Description

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• I've leverage the tools to beautify the python and c++ code.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted).
• All changes have test coverage
• Code is well-documented
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
• Related issue is referred in this PR
• If the PR is for a new model/paper, I've updated the example index here.

Changes

[dgl-bot]

To trigger regression tests:

• @dgl-bot run [instance-type] [which tests] [compare-with-branch];
  For example: @dgl-bot run g4dn.4xlarge all dmlc/master or @dgl-bot run c5.9xlarge kernel,api dmlc/master

[dgl-bot]

Commit ID: 3af0d2f3102e7bc91ae6b2a02f6b235ab9834a04

Build ID: 1

Status: ❌ CI test failed in Stage [Lint Check].

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Full logs path: link

[dgl-bot]

Commit ID: a5959f13d9bb93a85609d7ada60996396c29ecdd

Build ID: 2

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: db5c6197797174f3c837eb0ad66410c3b5058fae

Build ID: 3

Status: ❌ CI test failed in Stage [Lint Check].

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[dgl-bot]

Commit ID: 059d00cf5b2b27c884d96397139e5834a51f37ba

Build ID: 4

Status: ✅ CI test succeeded.

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[dgl-bot]

Commit ID: 855724bfe5fec96870d4321ddda38975b0565819

Build ID: 5

Status: ❌ CI test failed in Stage [Distributed Torch CPU Unit test].

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Full logs path: link

[dgl-bot]

Commit ID: 1eb97c8d8a4820ff79596120a3c00ee9ee91f513

Build ID: 6

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 95182f33a7e0f67e96ceafd82c242a3a725358c6

Build ID: 7

Status: ⚪️ CI test cancelled due to overrun.

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[dgl-bot]

Commit ID: 506c7d5afc83ec1005c3b423c0f3315a0bdee5a4

Build ID: 8

Status: ❌ CI test failed in Stage [Distributed Torch CPU Unit test].

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Full logs path: link

[dgl-bot]

Commit ID: 9615f543af6af2cdc9f95bde976a2ff26e465561

Build ID: 9

Status: ✅ CI test succeeded.

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Full logs path: link